The National Institute of Allergy and Infectious Diseases (NIAID) is supporting efforts to develop a recombinant subunit vaccine against the Category B agent ricin toxin. The current focus is on the toxin's 267- amino acid enzymatic A subunit (RTA). To date, vaccine design has been aimed at making point mutations and site-specific deletions in RTA to attenuate its enzymatic activity so that it is safe for use in humans. Very little attention has been paid to how these mutations may affect B-cell epitopes on RTA that are critical for eliciting protective immunity. It is known that neutralizing antibodies consttute only a very small fraction of the total antibody pool elicited by RTA immunization. The overwhelming antibody response is made up of non- neutralizing and toxin-enhancing antibodies. This fact may explain the relative ineffectiveness of the current lead vaccine candidate to elicit detectable serum neutralizing activity despite high titer anti-toxin antibodies Because serum neutralizing antibody titers are the singular correlate of immunity to ricin, it is essential that RTA-based subunit vaccine stimulate measurable serum neutralizing activities. Unfortunately, in the absence of a comprehensive B-cell epitope map of RTA it is not possible to engineer derivatives RTA in which key protective B-cell epitopes are preserved, while epitopes that give rise to non-neutralizing (or even deleterious) antibodies are eliminated. This applicatio proposes to generate a comprehensive and high-resolution B-cell epitope map of RTA. This will be accomplished using a unique collection of resources, including a large panel of RTA-specific mAbs, as well as a collection of RTA derivatives with mutations in all of the known immunodominant regions of the protein. The proposed research project is significant in that the resulting B-cell epitope map will be used in conjunction with the available structural information about RTA to design novel antigens in which targets of neutralizing antibodies are preserved and presented in a context designed to most effectively elicit protective immunity.